WHO: 'Diesel exhaust fumes cancerous'

Thursday June 14 2012

The Daily Mail reports a World Health Organization (WHO) warning that diesel exhaust fumes are a “major cancer risk” and belong in the “same deadly category as asbestos, arsenic and mustard gas”. Meanwhile the BBC says that diesel fumes are “definitely a cause of lung cancer”.

This widely reported news is based on a decision by the WHO to classify diesel exhausts as a cause of cancer.

The decision was taken by the WHO’s International Agency for Research on Cancer (IARC), a panel of experts that co-ordinates and conducts research into the causes of cancer, and develops cancer control strategies.

Under its classification scheme, diesel exhaust was previously categorised as “probably carcinogenic”. The agency now says there is now sufficient evidence that exposure to diesel fumes causes lung cancer. It is calling for exposure to diesel fumes to be reduced worldwide.

While diesel fumes are now officially carcinogenic, the alarmist tone of the Daily Mail’s headline should be viewed with caution because the ‘deadly category’ of substances the Mail describes also includes sunlight and wood dust.

What is diesel and is it used much in the UK?

Diesel oil is a complex mixture of chemicals, mainly distilled from crude oil, although vegetable oil and similar sources can be used to make ‘biodiesel’. It is used as fuel for diesel internal combustion engines, which use compressed hot air to ignite fuel (petrol engines have a spark plug to ignite the fuel).

Worldwide, diesel oil is widely used as a fuel in diesel-powered cars, lorries, trains, aircraft, ships and heavy industry. It is regarded as more efficient than petroleum, resulting in lower fuel consumption. WHO says that many people are exposed to diesel exhaust in everyday life, both through their occupations and in the ambient air.

As of 2007, just over 50% of all new car sales in the UK were diesel, according to the Society of Motor Manufacturers and Traders. In 2004, approximately 700 litres (150 gallons) of diesel was sold every second in the UK, according to a report by the Health Protection Agency (HPA).

The amount of pollutants from diesel exhaust fumes, in particular its sulphur content, have been reduced over the last few years, and engines on newer cars are designed to burn fuel more efficiently, reducing emissions. However, the IARC says it is not yet clear how these improvements translate into any changes in the impact of diesel fumes on human health. Existing fuels and older unmodified vehicles will take years to replace, particularly in less developed countries where regulations are less stringent, the IARC says.

What is the WHO’s classification scheme?

The WHO classifies the cancer-causing potential of various substances into four groups, depending on the evidence available in both humans and other animals:

  • Group 1 is used when a substance causes cancer in humans
  • Group 2A is used  when a substance ‘probably’ causes cancer in humans
  • Group 2B is used when a substance ‘possibly’ causes cancer in humans
  • Group 3 is used when a substance is not classifiable in terms of its cancer-causing properties in humans because the evidence is inadequate
  • Group 4 is used when a substance is ‘probably not’ a cause of cancer in humans

What is the WHO now saying about diesel fumes and cancer?

Since 1988, diesel oil fumes have been classified by the IARC as ‘probably carcinogenic to humans’. This category is used when there is some, limited evidence that a substance causes cancer in humans, but sufficient evidence it causes cancer in experimental animals.

However, the IARC has now reclassified diesel engine exhaust as ‘carcinogenic’ (group 1 on the list above). This category is used when there is sufficient evidence that a substance causes cancer in humans. The IARC says there is sufficient evidence that diesel exhaust is a cause of lung cancer. It is also associated with an increased risk of bladder cancer, although the evidence for the latter is more limited.

Why has the advice changed?

The WHO says there has been mounting concern in recent years about the cancer-causing potential of diesel engine exhaust, based on findings from epidemiological studies of workers exposed to diesel fumes. In particular, it cites a large cohort study, published in March this year, of occupational exposure to diesel exhaust in 12,315 US miners. The study was run by the US National Cancer Institute and the National Institute for Occupational Safety and Health. It found that exposure to diesel exhaust increased the risk of dying from lung cancer (1.26, 95% confidence interval 1.09 to 1.44). A further case-control study, undertaken in this group (comparing 198 miners who had died from lung cancer with 562 miners who were alive at the time the ‘case’ died), found that the risk of lung cancer in these workers increased with the length of time they were exposed to diesel fumes.

Although these studies were in workers who had been heavily exposed to diesel fumes, the WHO points out that studies of other carcinogens, such as radon, indicate that initial research showing a risk in heavily exposed populations, are later found to be a risk for the general population. It says that action to reduce exposure to diesel exhaust fumes should encompass both highly exposed workers and the general population.

Are diesel fumes really as dangerous as asbestos and mustard gas?

Under the IARC classification scheme, diesel fumes now fall into the same category as all other known carcinogens (of which there are over 100 listed). These include:

  • tobacco smoke (both first and secondhand)
  • mustard gas
  • sunlight
  • Chinese salted fish
  • vinyl chloride
  • soot
  • wood dust

The WHO does not specify the level of risk posed by different carcinogens, or the risk posed by different levels of exposure. However, for most carcinogens, the higher the exposure, the greater the risk of cancer.

Dr Christopher Portier, chair of the IARC working group, said that while the scientific evidence that diesel oil exhaust caused lung cancer was ‘compelling’, the impact on the wider population who are exposed to diesel fumes at much lower levels and for shorter periods of time, is unknown.

Newspaper readers of a nervous disposition may want to consider the above facts when considering the risk to their health from diesel fumes.

Analysis by Bazian
Edited by NHS Choices